This invention relates to vertical form, fill and seal machines and in particular to settling of product within a partially-formed package or bag in a continuous motion vertical form, fill and seal bag making machine.
For many years, manufacturers of vertical form, fill and seal machines have been concerned about seal quality. This is particularly important in the end seal area where a transverse seal is formed across the bag. In this seal area, there is the possibility for product to be trapped between the surfaces of the inside of the partially formed tube by sealing jaws when the jaws close to complete the seal. The trapped product keeps the film surfaces from achieving intimate contact for sealing, and the result is an open or partially open package. While this is a serious problem at any time, it is a particularly serious problem when the product being sealed by the machine is a food product subject to spoilage or contamination. When the package is to contain an inert gas for preservation of the product therewithin, it is extremely important that there be a complete hermetic seal.
One major cause of having product trapped in the seal area is the inability of the machine to settle product in the bag prior to closing the sealing jaws. Thus, stripping in the seal area has been developed, and many stripping devices have been developed over the years to ensure the product is not trapped in the seal area between the sealing jaws. However, if the product is fragile, conventional methods of stripping often are not acceptable, particularly if breakage of the product is of concern or the product does not stack without a considerable amount of void areas.
Therefore, many shaking devices have evolved over the years to avoid the mechanical problems and product breakage inherent in customary forms of stripping. The earliest of these devices was based on the concept of clamping a movable shaking device to the bag in a position over the product in the bag. After the bag pull has been completed and before the sealing jaws close, the shaker device, located above the sealing jaws, closes on the film over the unsettled product. Once closed, the shaking device raises and lowers the bag several times with adjustable amplitude and frequency to shake the bag and the product contained within the partially-formed bag in order to settle the product below the plane of the sealing jaws.
A major objection to this type of settling device is the amount of time it takes to clamp the bag and shake the bag prior to initiating the end seal. Thus, such a device is normally unusable in a high speed bag making process. In addition to the time requirement, there is also a possibility of bag deformation due to the clamping action of the shaker jaws on the film tube.
Another device developed for product settling has been a bag tamper, where rather than clamping to the bag, an exterior striking element is employed, impinging on the exterior of the bag, to strike the partially-completed bag and cause shaking in that manner. However, this device, in addition to being relatively slow to settle the product, also has the added disadvantage that striking the exterior of the tube can cause breakage of fragile product contained within the partially-formed bag, or deformation of the tube.
U.S. Pat. No. 5,485,712, assigned to the assignee of the present application, discloses a different means of product settling by actually reversing the direction of the film. The method and apparatus of this patent employs measuring rolls for metering the film, and pull belts for advancing the film after it has been formed into a tube. In accordance with one form of the patent, the film direction is reversed with the sealing jaws partially closed to provide a reverse stripping motion. In another form of the invention of the ""712 patent, the method of achieving product settling involves driving the film in forward and reverse directions using a programmed time and frequency to achieve the desired product settling. While this type of settling can be achieved with either a servo-driven system or a stepper motor system, it also can be inherently slow because of the necessity to reverse directions several times in order to settle product within the partially-formed bag.
Further problems occur when the film does not stop, but operates what is called a continuous motion. In many of the previous shaking or stripping devices, it is necessary for the film to be stopped when the shaking or stripping occurs. When, however, the film is proceeding with a continuous motion mode of operation, such methods are unusable.
The invention comprises a method and apparatus of settling product in a vertical form, fill and seal machine. The machine comprises a source of film, a forming shoulder for forming the film into a tube, a pull axis for conveying the tube, and a finishing system for sealing and severing successive completed packages from the tube. The method according to the invention comprises the steps of operating the pull axis in a forward direction to continually pull the film. A desired portion of product is then injected into the tube as the tube continues to advance, and an instability is induced in the pull axis for a predetermined period of time to shake the tube and settle the product while the tube and the film continue to advance.
In accordance with the preferred form of the invention, after the predetermined period of time, the finishing system is then activated and follows the film at matched speed while a sealing operation is completed. Until then, the finishing system is not activated in order to permit product to be properly settled from the seal area between the sealing jaws. After sealing, the sealing jaws are returned for the next cycle of operation.
In accordance with a preferred form of the invention, the pull axis includes a servo motor for operating the axis, and the step of inducing the instability in the pull axis comprises setting the gain of the servo motor to induce oscillation. The proportional or P gain of the servo motor can be increased in order to increase the amplitude of the oscillation. The frequency of oscillation is determined by the dynamics of the system.
In the preferred form of the invention, a measuring roll axis is included for conveying the film from the source of film and for measuring a predetermined amount of the film. A pull belt axis is used to assist the film over a forming shoulder for forming the film into a tube. In this form of the invention, the step of inducing an instability comprises inducing an instability in the pull belt axis while leaving the gain adjustment in the measuring roll axis in a stable condition. Thus, instability is induced only in the pull belt axis, with the measuring roll axis being unaffected by the induced instability. Thus, control of the film can always be maintained in the measuring roll axis, with pulling of the tube over the forming shoulder and shaking of the tube occurring due to action in the pull belt axis. In this manner, the control of a registered film can always be maintained to provide high quality packages with end seals of high integrity.